Perfluoroelastomers (elastomeric perfluoropolymers) exhibit outstanding high temperature tolerance and chemical resistance in both the cured and uncured states. These properties are attributable to the stability and inertness of the copolymerized perfluorinated monomer units which form the major portion of the polymer backbone, e.g. tetrafluoroethylene, perfluoro(methyl vinyl)ether, perfluoro(propyl vinyl)ether and others disclosed in U.S. Pat. Nos. 3,467,638; 3,682,872; 4,035,565; 4,281,092; and 4,972,038. Perfluoroelastomers also, however, necessarily contain small quantities of less stable copolymerized cure site monomers and, in addition, many perfluoroelastomers contain reactive end-groups introduced by the use of chain transfer agents or molecular weight regulators during polymerization. Such moieties must have a high degree of reactivity in order to promote effective crosslinking and cure chemistry, but this reactivity inherently renders the polymers more susceptible to degradative chemical reactions, such as oxidation. Consequently, certain physical properties of the polymer, in particular compression set, and high temperature stress/strain properties, are adversely affected. For example, the presence of copolymerized hydrogen-containing cure site monomers often results in decreased thermal stability. Other factors can also influence thermal stability of perfluoroelastomer compositions. As an example, the use of peroxide cure systems which incorporate hydrocarbon coagents, such as triallylisocyanurate, limits the upper service temperature of cured perfluoroelastomer compositions.
Thus, even the most stable of the previously-known perfluoroelastomer compositions suffer unacceptable degradation of properties in certain environments which are chemically and thermally demanding. For example, perfluoroelastomers containing pentafluorophenoxy or cyano-substituted perfluoro(vinyl ether) cure site monomers, which are among the most stable, cannot withstand long exposure to oxidative environments at temperatures exceeding 280.degree. C. or contact with hydrocarbon fuel while under compressive stress at temperatures approaching 280.degree. C.
It has now however been found that thermal stability of certain perfluoroelastomers can be substantially enhanced by compounding the polymers with specific amounts of large particle size, low surface area carbon blacks. By employing appropriate amounts of these fillers, perfluoroelastomer compositions have been prepared which are suitable for extended use at temperatures of 280.degree. C. or more.